The present invention relates to production and purification of diborane. Diborane (B2H6) is a flammable gas which is used as a p type dopant in semiconductors, and is also used in boron-phosphate-silicate glass forming. Diborane forms a wide variety of complexes with lewis bases such as borane-tetrahydrofuran, borane dimethyl sulfide and a variety of amine boranes. These compounds are widely used as selective reducing agents in synthesis of pharmaceuticals, fine organic chemicals and electroless metal plating baths.
At room temperature, diborane slowly decomposes to higher boranes with their physical state ranging from gaseous to solid. This causes process variations and equipment malfunctions. In order to reduce decomposition, diborane is sometimes shipped as a mixture with a blanket gas or at low temperature, such as at dry ice temperature. Another way to overcome the decomposition problem is to employ point-of-use diborane generation. However, the difficulties encountered with present synthesis and purification processes have inhibited point-of-use diborane generation.
Numerous possible methods of diborane synthesis have been published. The most typical and commercially used synthesis method is the reaction of sodium borohydride with boron trifluoride in ether solvents such as diglyme. Because this process uses highly inflammable solvents, it requires significant safety precautions. Further, diborane complexes with solvents. Such complexes make it difficult to purify the diborane.
A preferred dry process for diborane synthesis is described in U.S. Pat. No. 4,388,284. This process involves reaction of lithium or sodium borohydride with boron trifluoride (BF3) in the absence of a solvent. As a preferred method, the patent describes condensing gaseous boron trifluoride at liquid nitrogen temperature onto sodium borohydride, then warming the resultant mixture to a reaction temperature of 0 to 50xc2x0 C. and holding the mixture at the reaction temperature for 4 to 12 hours. The process yields a mixture containing about 95% diborane and also containing unreacted boron trifluoride. Under similar conditions, reaction of lithium borohydride with boron trifluoride is sluggish and gives poor yield.
While the dry process provides diborane free from solvent contamination, the product contains significant amount of unreacted boron trifluoride. To achieve high purity diborane, tedious distillation is required to separate the diborane from the boron trifluoride. The process is slow for commercial production and is a batch process . Based upon thermodynamic considerations, the reaction of lithium borohydride with boron trifluoride should be more favored than the comparable reaction with sodium borohydride, but the observations set forth in the ""284 patent indicate that the reaction involving lithium borohydride does not work well in practice.
One aspect of the present invention provides methods for treating mixtures containing diborane and boron trihalides such as boron trifluoride by contacting the mixture with a reagent composition including one or more inorganic hydroxides. This aspect of the invention incorporates the discovery that inorganic hydroxides selectively scavenge boron trihalides, and particularly BF3, from gas mixtures containing diborane. For example, the reagent composition may include one or more alkali metal hydroxides such as sodium, potassium or lithium hydroxides; alkaline earth hydroxides such as beryllium, calcium, strontium and barium hydroxides; ammonium hydroxide; and transition metal hydroxides. Mixtures of these materials may be employed. Desirably, the reagent composition includes a substantial amount of the inorganic hydroxide, i.e. more than 10%, and preferably more than 20% hydroxides. Preferably, the composition is predominantly composed of the hydroxide or hydroxides, i.e., the reagent composition contains more than 50% mole fraction hydroxides. Most preferably, the reagent composition consists essentially of the hydroxide or hydroxides. The reagent composition typically is present in solid form, such as powder, pellets, granules or a coating on an inert support such as alumina or silica. The reagent composition may be pretreated by holding it at an elevated temperature prior to use, as, for example, by baking in an inert atmosphere prior to use.
The temperature in the contacting step preferably is room temperature (about 20xc2x0 C.) or below, and more preferably about 0xc2x0 C. or below. Temperatures below about xe2x88x9220xc2x0 C., and desirably below about xe2x88x9240xc2x0 C., are even more preferred. The use of such low temperatures minimizes decomposition of diborane in the process. Most preferably, the diborane-containing mixture is in the gaseous state when contacted with the reagent composition. Therefore, the temperature in the contacting step desirably is above the boiling temperature of diborane at the pressure employed. Stated another way, the prevailing pressure in the contacting step is below the equilibrium vapor pressure of diborane at the temperature employed for the contacting step. The boiling temperature of diborane is about xe2x88x9292xc2x0 C. at atmospheric pressure, and hence the temperature in the contacting step desirably is above about xe2x88x9292xc2x0 C. if the contacting step is performed at about atmospheric pressure. Dry ice temperature (about xe2x88x9280xc2x0 C.) is particularly preferred.
The time of contact between the mixture and the reagent composition may be a few seconds to a few hours, although very short contact times of a few seconds are more preferred. The contacting step can be performed batchwise or, preferably, on a continuous basis, by passing the mixture continuously through a vessel containing the reagent composition. The flow rate through the vessel, and the proportions of the vessel and amount of reagent composition can be selected to provide any desired contact time. Desirably, the purifying process, and particularly the contacting step, are performed at a location where the purified diborane is to be used, and the diborane is purified about 4 hours or less before it is used. Most preferably, the diborane is purified immediately before it is used.
Although this aspect of the invention has been summarized above in connection with purification of diborane, the process also can be applied to purification of other inorganic hydrides, and removal of inorganic halides other than boron trihalides such as BF3. Thus, process is applicable to remove inorganic halides from inorganic hydrides selected from the group consisting of diborane, silane (SiH4), germane (GeH4), phosphine (PH3), arsine (AsH3), stibine (SbH3) and mixtures thereof. Desirably, the inorganic halides which is or are removed are selected from the group consisting of BF3, SiF4, GeF4, PF3, PF5, AsF3, AsF5, SbF3, SbF5 and mixtures thereof.
A further aspect of the invention includes the realization that contacting the diborane-containing mixture with a hydroxide-containing reagent mixture will also serve to remove carbon dioxide if carbon dioxide is present in the mixture. Thus, processes according to this aspect of the invention include the steps of contacting a mixture containing diborane or other inorganic hydride as discussed above and carbon dioxide with a hydroxide-containing reagent. The process conditions may be as discussed above in connection with removal of halides. Where the gas mixture contains both halides and carbon dioxide, both can be removed in a single contacting step.
Yet another aspect of the invention provides methods of synthesizing diborane comprising reacting a borohydride reactant including potassium borohydride (KBH4) with a boron trihalide, most preferably BF3, to thereby form a reaction product. The reaction desirably is performed at a reaction temperature of about xe2x88x92130xc2x0 C. to about 20xc2x0 C. The reactant desirably includes at least 20% potassium borohydride, and preferably consists essentially of potassium borohydride or includes potassium borohydride together with sodium borohydride (NaBH4). The reacting step desirably is performed in the absence of a solvent and thus is referred to herein as a xe2x80x9cdryxe2x80x9d process. The reaction desirably is performed by continuously passing the boron trihalide, in gaseous form, as by passing the boron trihalide through a vessel containing the borohydride reactant in solid form. The reaction can also be performed in batchwise fashion, as by condensing the borohalide on the reactant in a vessel and then warming the vessel, reactant and borohalide. This aspect of the invention incorporates the realization that higher conversion of boron trifluoride to diborane is achieved by reacting it with potassium borohydride than with either lithium or sodium borohydride. The reaction with potassium borohydride is especially favored at the preferred temperatures of about xe2x88x92130 to about 20xc2x0 C. Most desirably, the reaction conditions are selected so that liquid BF3 is present in contact with the borohydride reactant during at least part of the reaction. Thus, BF3 desirably condenses on the borohydride reagent.
Still further aspects of the invention provide apparatus for performing the processes discussed above. Thus, one aspect of the invention provides a purifier to selectively scavenge inorganic halides such as BF3 and carbon dioxide from diborane-containing or other inorganic hydride mixtures, and also provides a diborane generation system including such a purifier. Another aspect of the invention provides a generator for making diborane using the potassium borohydride reaction discussed above, which may also include a purifier as discussed above. Apparatus according to these aspects of the invention may be installed at the point of use, and desirably is connected directly to diborane-using process equipment for continuous or batchwise transfer of the diborane made or purified in the apparatus into the diborane-using equipment.